ashtech GG Surveyor, Reference Manual

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Advanced Operation 41
Advanced Operation
After issuing the $PASHS,DTM,USR command, the receiver internally transforms positions from
the reference datum (WGS-84) to the user-defined datum. In standard text books, however, the
datum transformations are given from local datums to WGS-84. To simplify entering the
transformation parameters, the translation, rotation, and scale parameters are defined from the local
datum to WGS-84.
The generic formula used to translate and rotate from coordinate system 1 to
coordinate system 2 is as follows:
where
ε
rx
=
ε
x
expressed in radians, similarly for
ε
ry
and
ε
rz
.
Example: Define local datum as the WGS-72 datum
$PASHS,UDD, 0,6378135.0, 298.26,0,0,4.5,0,0,-0.554,0.23
$PASHS,DTM,USR
This implements the transformations listed in Table 4.7 and below.
∆
x=
∆
y=0
∆
z= 4.5 meters m= 0.23 x 10
-6
ε
x
=
ε
y
=0
ε
z
= –2.686 x 10
-6
radians = –0. 554
in the following equation:
Table 4.7: Ellipsoid Parameters for WGS-72 and WGS-84
Datum Reference Ellipsoid a[m] 1/f
WGS-72 WGS-72 6378135.0 298.26
WGS-84 WGS-84 6378137.0 298.257223563
x
y
z
2
∆
x
∆
y
∆
z
1 m
6 –
×
10 +
( )
1
ε
rz
ε
ry
–
ε
rz
– 1
ε
rx
ε
ry
ε
rx
– 1
x
y
z
1
+ =
x
y
z
WGS84
0
0
4.5
1 0.23
6 –
×
10 +
( )
1 2.686 –
6 –
×
10 0
2.686
6 –
×
10 1 0
0 0 1
x
y
z
1WGS72
+ =